Heavy petroleum oil and residual oil having a high content of sulfur such as petroleum asphalt are less valuable because of serious impact on the environment when directly burnt as fuel. Consequently, such heavy petroleum oil (including residual oil; hereinafter the same) is cracked and converted into lighter fractions to be used as various kinds of useful industrial taw materials. As one of such methodologies, the following thermal cracking process is mentioned.
As a process for thermal cracking of heavy petroleum oil, there is a proposed process in which a thermal cracking facility is provided with a cracking furnace and a reaction vessel and the heavy petroleum oil heated in the cracking furnace is introduced into the reaction vessel to be thermally cracked (refer to Patent Document 1). In the thermal cracking process, the heavy petroleum oil is introduced into the reaction vessel after being heated to the higher temperature than 450° C. in the cracking furnace so that the conversion rate of cracking reaction reaches 30 to 45%. The heavy petroleum oil fed into the reaction vessel is directly contacted with superheated steam of 500 to 700° C. that is blown in from the bottom of the reaction vessel and thermally cracked to produce gaseous cracked substances as cracked product. The cracked product obtained by this process is gaseous cracked substances rich in aliphatic hydrocarbons as a main product and petroleum pitch rich in poly-aromatics as a by-product. The gaseous cracked substances produced are discharged together with steam from the upper discharge outlet of the reaction vessel and introduced into the distillation tower provided for distillation separation of them.
FIG. 14 shows the schematic perspective view of a reaction vessel 106 provided for a conventional thermal cracking process for heavy petroleum oil. As shown in FIG. 14, the reaction vessel 106 comprises a main body portion (or a shell) 116 having a vertical cylindrical shape and a bottom portion (or a tapered bottom) having a tapered shape where is provided with blowing nozzles 114 connected to the inside of the reaction vessel 106. Superheated steam is blown into the inside of the reaction vessel 106 from the blowing nozzles 114 under the condition that the reaction vessel is filled with heavy petroleum oil. The role of the superheated steam blown in from the bottom of the reaction vessel is the heating of heavy petroleum oil and the rapid discharge of cracked products.
In a conventional thermal cracking process of heavy petroleum oil, superheated steam is blown in through the blowing nozzles 114 to the slightly upward direction (the direction C′ as shown by arrows) against the central axis (dashed line S′ as shown in the drawing) of the shell 116 of the reaction vessel 106 and it generates great driving force to the direction E as shown by the arrow. However, preferable uniform dispersion of superheated steam can not be realized since this behavior of the superheated steam causes blow-through to the center of the reaction vessel 106 and partially stagnant portion of heavy petroleum oil inside of the reaction vessel 106. When uniform dispersion of superheated steam is not realized, it causes remarkable degradation of pitch quality due to higher content of quinoline-insoluble component as well as increased coking tendency due to uneven heating and delay in the discharge of the cracked products. Further, there is severe entrainment of pitch caused by blow-through and the adherence of coke inside the downstream cracked gas piping is likely to be occurred, and it may cause plugging of piping.    [Patent Document 1] JP07-116450 B (1995)    [Patent Document 2] JP54-15444 B (1979)    [Patent Document 3] JP57-15795 B (1982)    [Patent Document 4] JP63-38076 B (1988)